1. Field of the Invention
The present invention relates to a grid nozzle assembly for a fluidized bed reactor, a fluidized bed reactor with a grid nozzle assembly, a method of mounting a grid nozzle assembly as a replacement in a fluidized bed reactor, and a method of replacing a grid nozzle assembly in a fluidized bed reactor.
2. Description of the Related Art
In fluidized bed reactors, such as circulating or bubbling fluidized bed boilers or gasifiers, chemical reactions are performed in a reaction chamber in a fluidized bed of particles, such as sand or ash or fuel particles. The bed is fluidized by bringing fluidizing gas, such as combustion air, to the reaction chamber from a gas plenum chamber, usually so-called a wind box, through a bottom grid of the reaction chamber. The bottom grid comprises grid nozzles through which the gas is injected in a predetermined, relatively high velocity and predetermined direction to the reaction chamber. The grid nozzles can also be used for removing solid particles from the bottom of the reaction chamber.
The conditions in the reaction chamber are often relatively harsh due to high temperatures, chemical attack of compounds formed in the reaction chamber, or erosion caused by bed particles entrained with the fluidizing gas. Due to the harsh conditions, there is often a need to regularly replace grid nozzles by new ones. The replacement has to be done during relatively short outage times of the fluidized bed reactor. Because there can be several thousands of grid nozzles in the bottom grid of a fluidized bed reactor, the easiness of the replacement of the grid nozzles is of great importance.
Commonly used grid nozzles of fluidized bed reactors are made of steel and comprise a vertical tube or gas pipe that is welded to a bottom plate of the reaction chamber and a head portion, so-called a nozzle head, through which the fluidizing gas is guided horizontally or slightly downwards to the fluidized bed reactor either as evenly distributed in all directions or directed in a certain direction. The bottom plate and a lower portion of the vertical tubes are usually covered by a protecting refractory layer. During the operation of the fluidized bed reactor, a portion of the vertical tubes may also be embedded in stagnant or slowly moving bed particles, whereas the head portions are directly exposed to the harsh conditions. Thus, the nozzle heads are especially prone to deterioration. Also, it is typical to fluidized bed reactors that some ash may enter in the fluidizing gas being supplied to the fluidized bed reactor. Due to the ash, there is a risk for high velocity ash erosion to occur inside the grid nozzles, increasing the need for frequent maintenance. Thus, there is a need for grid nozzles, the nozzle heads of which can be easily replaced.
In conventional grid nozzles, such as those shown in Canadian Patent No. 1151474, the vertical tube and the nozzle head are of one piece, or they are welded together so as to form a monolithic, integral piece. Thus, when there is a need to replace the nozzle head, it is common practice to cut the vertical tube and to weld a replacement part with a new nozzle head around the upper end of the remaining portion of the vertical tube. This is a relatively slow process, which tends to shorten each time the remaining tube. It also gives rise to a considerable amount of harmful metal debris, which has to be removed from the bottom grid.
U.S. Pat. No. 3,708,887 shows a grid nozzle in which the vertical tube comprises an external thread and a tubular member of a nozzle head comprises a corresponding internal thread so as to connect the nozzle head to the vertical tube. Such a threaded nozzle head coupling is not suitable for many large scale fluidized bed reactors, because the thread is relatively slow to open, and it may easily become stuck during the operation of the reactor.
International Patent Publication No. WO 2010/011457 A2 shows a grid nozzle, in which, around the upper end of a vertical tube, is welded a sleeve with an internal thread, and a lower portion of a tubular nozzle head comprises a corresponding external thread so as to connect the nozzle head to the sleeve. The connection of the tubular nozzle head to the sleeve is secured by tack welds between an outer rim of the nozzle head and the upper edge of the sleeve. This nozzle grid construction is also relatively slow to open and may deteriorate or become stuck during the operation.
An object of the present invention is to provide a new grid nozzle assembly that at least partially minimizes problems of the prior art described above.